ylides

Also contains definition of: nitrogen ylides

Compounds in which an anionic site
Y−
(originally on carbon, but now including other atoms) is attached directly to a heteroatom
X+ (usually nitrogen, phosphorus or sulfur)
carrying a formal positive charge.
They are thus 1,2-dipolar species of the type
RmX+–Y−Rn.
If X is a saturated atom of an element from the first row of the periodic system,
the ylide is commonly represented by a charge-separated form; if X is a second, third,
etc. row element uncharged canonical forms are available
RmX=YRn. If X is an unsaturated atom, doubly bonded to another first row element Z, the negative
charge on Y may be stabilized by
π-conjugation,
Z=X+–Y−RnZ−–X+=YRn.
Such ylides belong to the class 1,3 dipolar compounds.
However, 1,3-dipolar compounds with only sextet-containing canonical forms
(e.g. vinylcarbenes ) are not ylides. E.g.
Ph3P+–C−H2Ph3P+=CH2
(often called a Wittig reagent),
(CH3)3N+–C−H2,
RC≡N+N−–R,
(CH3)2S=CHPh(CH3)2S+–C−HPh.
Note that ylide is a complete word, not to be confused with the suffix -ylide, used
for some radical anions. Subclasses of ylides: Ylides
RmX+–C−R2
having the negative charge on carbon are classified by citing the name of the element
X before the word ylide.
E.g.
nitrogen ylide,
phosphorus ylide,
oxygen ylide,
sulfur ylide.
A further specification may be achieved by citing the class name of
RmX
before the word ylide.
Thus nitrogen ylides include amine ylides,
R3N+–C−R2,
azomethine ylidesR2C=N+R–C−R2,
nitrile ylides,
RC≡N+–C−R2.
Some authors, who wish to express the positive charge on X, prefer e.g.
ammonium ylides over amine ylides; such usage varies
according to the heteroatom X and to national custom.
The ylides
RmX+–Y−RmX=Y
(Y
=
O,
S,
Se,
Te,
NR)
are usually named by citing the name of
RmX
followed by
the additive nomenclature term for
Y
(oxide, sulfide, selenide, telluride, imide, respectively). E.g. amine imides; use of the less systematic synonyms amine imines and aminimines is discouraged. Some classes of ylides are known by trivial names
e.g. nitrones, Wittig reagents (synonymous with phosphonium ylides).